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On Earth, carbonate minerals are widely used as recorders of the geological environments in which they formed. Here, we present a method designed to retrieve the crystal chemistry of Fe‐Mg carbonate minerals using infrared remote sensing or Raman spectroscopy. We analyzed a suite of well‐characterized Fe‐Mg carbonate minerals for which Raman spectra were obtained in two different laboratories, and IR spectra were measured in reflectance and transmission from the visible range to 25‐μm. We built calibration lines for the dependence of fundamental and harmonic vibrational modes position to the Mg# (defined as Mg# = 100 × Mg/(Mg + Fe + Ca + Mn)). These calibrations should enable retrieval of Mg# based on spectroscopic observations with a typical accuracy of 10. We discuss the framework of applicability of these calibrations and apply them to a typical CRISM spectrum of carbonates from the Nilli Fossae region of Mars. Key Points: Fundamental FeMg carbonate mode position have a linear dependence to Mg# for Fe‐Mg carbonate mineralHarmonic bands at 2.3 and 2.5 also have a linear dependence of their position with Mg# for Fe‐Mg carbonate mineralsWe present calibration curves of Mg# versus fundamental and harmonic vibration position than can be used to retrieve crystal chemistry [ABSTRACT FROM AUTHOR]